Catalytic layers for fuel cells are well known which comprise an ionomer, e.g., persulfonated polytetrafluoroethylene, such as that sold as NAFION®; conductive particulates, typically carbon; and catalyst that is typically supported by the conductive particulates. Catalytic materials include very fine particles of catalytic metals, such as platinum, gold, ruthenium, palladium, and combinations of such materials. For a fuel cell that uses hydrogen or a hydrocarbon as the fuel and oxygen as the oxidant, the cathodic catalytic layer typically uses platinum as the catalyst and the anodic catalytic layer typically uses a platinum/ruthenium mixture as the catalyst. Typically, the carbon particulates have mean particulate diameters of between 20 and 2000 nanometers; while the catalytic particulates supported by the carbon particulates typically have mean particulate diameters of between 2 and 20 nanometers. Catalytic layers may also employ particulates of a hydrophobic material, such as polytetrafluoroethylene (PTFE, such as that sold as TEFLON®) for water management.
Because fuel cells run on gases, it is implicit that the catalytic layers of fuel cells have sufficient porosity to be permeable to gases. The pore sizes should be large enough to allow passage of the gas into the layer, but sufficiently small such that substantially all of the gas contacts a catalytic surface in its passage through the layer so as to chemically react and produce electrons or create an electron deficit, as the case may be.
A prior art method of providing a catalytic layer is to produce a fluid composition, e.g., a solution/suspension, of the layer components, e.g., carbon-supported catalyst dispersed in a solution/suspension of NAFION® (persulfonated polytetrafluoroethylene); apply, e.g., with doctor blades, this fluid to a substrate surface, e.g., the surface of a woven or non-woven carbon cloth; and subsequently dry the fluid to produce the catalytic layer. The surface of the substrate, which comprises a carbon cloth, has an uneven surface. When a layer is formed from a solution and a doctor blade is used in application of the solution, the surface of the catalytic layer away from the substrate is substantially smooth. As the surface of the catalytic layer that contacts the uneven substrate is similarly uneven, the catalytic layer that is formed is of uneven thickness. All portions of the catalytic layer must be sufficiently thick to ensure that substantially all of the gas passing through the layer comes into contact with catalyst. This means that portions of the layer are thicker than is required. As a result, catalytic material, including expensive catalytic metal, is used in excess if applied by this method. Furthermore, thicker portions of a catalytic membrane are less efficient in transporting electrons and in transporting chemicals, such as water, that must be removed from the layer.
U.S. Pat. No. 5,861,222 describes a method of forming a catalytic layer by spraying a catalytic material fluid containing ionomer and catalyst, such as carbon-supported platinum, on a substrate surface and subsequently drying the fluid to form a layer. A layer that forms by this method is denser than desired. To achieve the porosity required for the catalyst layer to function, i.e., transport of gases and other chemicals through the layer, the catalytic material fluid described in U.S. Pat. No. 5,861,222 includes a “pore forming material” such as carbonates and bicarbonates that are subsequently removed by treating the layer with acid, such as sulfuric acid, so as to develop the requisite porosity.
U.S. Pat. No. 6,403,245 issued 11 Jun. 2002, describes a method of forming catalytic fuel cell layers by co-depositing catalytic particulates, e.g., platinum particulates, formed by combustion chemical vapor deposition (CCVD) and at least a solution containing NAFION (persulfonated polytetrafluoroethylene) and/or suspended carbon particulates. The layers produced by the method of U.S. Pat. No. 6,403,245 may be thin and conformal. By “conformal” is meant that the deposited catalytic layers conform in contours to the uneven contours of the substrate on which the layer is deposited, both the surface of the layer on the substrate and the opposite surface of the layer generally conforming in contour to that of the substrate. Such conformal layers exhibit generally uniform thickness that contributes to the efficiency of the layer. While the conformal nature of the layer that forms contributes to the catalytic efficiency of the layer, deposition of layers as described in U.S. Pat. No. 6,403,245 has difficulties with high speed, large-scale deposition of catalytic materials and does not account for a variable permeable substrate.
Critical to the operation of the fuel cell, whether a PEFC or a DMFC, is the proton-conducting membrane. The present invention is further directed to proton-conducting membranes that provide unique properties or utilities and the method of their manufacture.
General properties of proton-conducting membranes are that they exhibit ionic exchange capacity, proton conductivity, thermal stability, mechanical stability, and chemical stability, particularly electrochemical stability, i.e., the chemical components are neither reduced nor oxidized during constant cycling. Low cost is also an important consideration in proton-conducting membranes.
The present invention, therefore, is directed to methods and apparatus for providing porous catalytic layers with high levels of catalyst and high effective catalytic surface area of the catalyst and to proton-conducting membranes that provide unique properties or utilities and the method of their manufacture. The method of the present invention provides for efficient, large scale deposition of catalytic material to form catalytic layers.